Conventionally, to enhance friction on ice of studless tires, rubber compositions for tires containing thermally expandable microcapsules have been developed.
For example, Patent Document 1 discloses “a rubber composition for a tire tread, for a tire bead filler, or for a reinforcing liner of a run flat tire, the rubber composition comprising 100 parts by weight of a diene rubber and 0.5 to 25 parts by weight of a heat expandable microcapsule composed of a thermoplastic resin particle having a substance enclosed therein, the substance being capable of vaporizing or expanding due to heat to thereby generate a gas, wherein a shell of the heat expandable microcapsule comprises a thermoplastic resin obtained by polymerization of a nitrile-based monomer (I) as a main component of a monomer, a monomer (II) having an unsaturated double bond and carboxyl group in the molecule thereof, a monomer (III) having two or more polymerizable double bonds and, optionally, a copolymerizable monomer (IV) for adjusting expansion characteristics”.